(1) Field of the Invention
This invention relates to a constant flow rate expansion valve, and more particularly to a constant flow rate expansion valve for a refrigeration cycle of an automotive air conditioner, for changing high-temperature and high-pressure refrigerant into low-temperature and low-pressure refrigerant to deliver the resulting refrigerant to an evaporator at a constant flow rate.
(2) Description of the Related Art
In a refrigeration cycle of an automotive air conditioner, when a differential pressure control valve for providing control such that the differential pressure across a compressor or the suction pressure is constant is used for a control valve for controlling the compressor, to stabilize controllability of the system, it is considered to be desirable to use a constant flow rate expansion valve different in control method, for an expansion valve. A constant flow rate expansion valve of this kind is known e.g. from Japanese Unexamined Patent Publication No. 2001-153495.
The constant flow rate expansion valve includes a constant flow rate mechanism which is based on a principle that if the cross-sectional area of a passage through which refrigerant flows between a refrigerant inlet and a refrigerant outlet, and a differential pressure across the passage are determined, it is possible to make constant the flow rate of refrigerant flowing through the expansion valve, and control the flow rate to a constant flow rate corresponding to a value set by a solenoid by varying one of the cross-sectional area and the differential pressure by the solenoid. More specifically, the constant flow rate mechanism includes a flow path cross-sectional area control valve for controlling the cross-sectional area of a passage, and a constant differential pressure valve for making constant the differential pressure between the inlet and outlet of the flow path cross-sectional area control valve, and controls the flow path cross-sectional area of the flow path cross-sectional area control valve by a solenoid, to thereby hold the flow rate of refrigerant flowing through the expansion valve at a predetermined constant flow rate corresponding to the flow path cross-sectional area set by the solenoid (see FIG. 1 in Japanese Unexamined Patent Publication No. 2001-153495). Alternatively, the constant flow rate mechanism includes a restriction passage having a fixed cross-sectional area, and a differential pressure control valve for making constant the differential pressure between the inlet and outlet of the restriction passage, and controls the differential pressure set to the differential pressure control valve by a solenoid, to thereby hold the flow rate of refrigerant flowing through the expansion valve at a predetermined constant flow rate corresponding to the differential pressure set by the solenoid (see FIG. 2 of Japanese Unexamined Patent Publication No. 2001-153495).
In the conventional constant flow rate expansion valve, however, the type of controlling the flow path cross-sectional area is configured such that a pressure-sensing section of the constant differential pressure valve slides between the refrigerant inlet and the refrigerant outlet so as to be able to open and close the constant differential pressure valve by sensing an intermediate pressure between the constant differential pressure valve and the flow path cross-sectional area control valve, and a pressure at the outlet. Further, the type of controlling the differential pressure is also configured such that a pressure-sensing section of the differential pressure control valve slides between the refrigerant inlet and the refrigerant outlet so as to be able to open and close the differential pressure control valve, by sensing an intermediate pressure between the differential pressure control valve and the restriction passage, and a pressure at the outlet. Therefore, in both of the types, there occurs leakage of refrigerant from the refrigerant inlet to the refrigerant outlet through respective sliding portions, and it is difficult, for example, to control the flow rate of refrigerant substantially to zero, even if the control of the flow rate to zero is attempted.